Vertical polysilicon-germanium heterojunction bipolar transistor

ABSTRACT

A vertical heterojunction bipolar transistor (HBT) includes doped polysilicon having a doping of a first conductivity type as a wide-gap-emitter with an energy bandgap of about 1.12 eV and doped single crystalline Ge having a doping of the second conductivity type as the base having the energy bandgap of about 0.66 eV. Doped single crystalline Ge having of doping of the first conductivity type is employed as the collector. Because the base and the collector include the same semiconductor material, i.e., Ge, having the same lattice constant, there is no lattice mismatch issue between the collector and the base. Further, because the emitter is polycrystalline and the base is single crystalline, there is no lattice mismatch issue between the base and the emitter.

RELATED APPLICATIONS

The present application is related to co-assigned and co-pending U.S. application Ser. No. ______ (Attorney Docket No: YOR920110110US1; SSMP 27302).

BACKGROUND

The present disclosure relates to semiconductor devices, and particularly to vertical polysilicon-germanium heterojunction bipolar transistors and methods of manufacturing the same.

The collector current of a bipolar transistor is a function of the energy bandgap of the base region material. For a regular silicon base bipolar transistor, it takes an emitter-base bias of about 0.92 V to obtain a collector current density of 10 mA/μm². It is desirable to be able to reduce the voltage needed to drive a bipolar transistor.

The state of art SiGe-base bipolar transistors have silicon in the collector, a SiGe alloy in the base, and polysilicon as emitter. The distribution of Ge across the SiGe base region can be relatively uniform, or can be graded with higher Ge concentration near the collector end and lower, or zero, Ge concentration at the emitter end.

An example of the graded Ge distribution across the base region with zero Ge concentration at the emitter end can be found in the paper by D. L. Harame et al., “Si/SiGe epitaxial-base transistors—Part I: materials, physics, and circuits,” IEEE Transactions on Electron Devices, vol. 42, p. 455, 1995. The SiGe base region is single crystalline, and is grown epitaxially on top of a single crystalline silicon collector layer. With zero or relatively low Ge concentration at the emitter end, there is little energy bandgap difference between the emitter and the base at the emitter-base junction. Thus, this transistor is not a wide-gap-emitter heterojunction bipolar transistor.

An example of a SiGe-base bipolar transistor having relatively uniform Ge distribution across the base region can be found in the paper by P. Deixler et al., “Explorations for high performance SiGe-heterojunction bipolar transistor integration,” Proceedings of BCTM, p. 30, 2001. The SiGe base region is single crystalline, and is grown epitaxially on top of a single crystalline silicon collector layer. With the Ge distribution that is relatively uniform across the base, the energy bandgap of the base region is smaller than the energy bandgap of the emitter at the emitter-base junction. While this is a wide-gap-emitter heterojunction bipolar transistor, the Ge concentration in this prior art is only 15%, resulting in an energy bandgap of about 1.0 eV. Further increase in the Ge concentration in the SiGe alloy in the base region is not possible because Ge concentration greater than 15% in a SiGe alloy destroys epitaxial alignment between the base and the collector due to excessive lattice mismatch between the underlying silicon material and the SiGe alloy. In other words, to avoid defects being generated in the SiGe-base region, the Ge concentration has to stay below some limit, i.e., at about 15% Ge or less.

BRIEF SUMMARY

In the present disclosure, a vertical heterojunction bipolar transistor (HBT) includes doped polysilicon having a doping of a first conductivity type as a wide-gap-emitter with an energy bandgap of about 1.12 eV and doped single crystalline Ge having a doping of the second conductivity type as the base having an energy bandgap of about 0.66 eV. Doped single crystalline Ge having a doping of the first conductivity type is employed as the collector. Because the base and the collector include the same semiconductor material, i.e., Ge, having the same lattice constant, there is no lattice mismatch issue between the collector and the base. Further, because the emitter is polycrystalline and the base is single crystalline, there is no lattice mismatch issue between the base and the emitter. The small band gap of the base enables reduction of the emitter-base bias by a factor of about 2 compared to a regular silicon-base bipolar transistor.

The vertical heterojunction bipolar transistor structure can be built using a bulk Ge substrate or a germanium-on-insulator (GOI) substrate. An npn vertical HBT or a pnp vertical HBT can be formed depending on the selection of the first conductivity type and the second conductivity type.

According to an aspect of the present disclosure, a heterojunction bipolar transistor (HBT) structure is provided, which includes: a collector including a first single crystalline germanium portion having a doping of a first conductivity type; a base including a second single crystalline germanium portion epitaxially aligned to the collector and having a doping of a second conductivity type that is the opposite of the first conductivity type; and an emitter including a polycrystalline silicon portion having a doping of the first conductivity type.

According to another aspect of the present disclosure, a method of fabricating a heterojunction bipolar transistor (HBT) structure is provided, which includes: forming a collector and a base in a substrate, wherein the collector includes a first single crystalline germanium portion having a doping of a first conductivity type and the base includes a second single crystalline germanium portion epitaxially aligned to the collector and has a doping of a second conductivity type that is the opposite of the first conductivity type; and forming an emitter including a polycrystalline silicon portion having a doping of the first conductivity type directly on the base.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a vertical cross-sectional view of a first exemplary structure according to a first embodiment of the present disclosure after formation of a subcollector layer, a collector layer, and a base layer.

FIG. 2 is a vertical cross-sectional view of the first exemplary structure according to the first embodiment of the present disclosure after formation of deep trench isolation structures.

FIG. 3 is a vertical cross-sectional view of the first exemplary structure according to the first embodiment of the present disclosure after formation of shallow trench isolation structures.

FIG. 4 is a vertical cross-sectional view of the first exemplary structure according to the first embodiment of the present disclosure after formation of a subcollector feedthrough region.

FIG. 5 is a vertical cross-sectional view of the first exemplary structure according to the first embodiment of the present disclosure after formation of an extrinsic base layer and an extrinsic base cap layer.

FIG. 6 is a vertical cross-sectional view of the first exemplary structure according to the first embodiment of the present disclosure after lithographic patterning of an extrinsic base and an extrinsic base cap.

FIG. 7 is a vertical cross-sectional view of the first exemplary structure according to the first embodiment of the present disclosure after formation of dielectric spacers.

FIG. 8 is a vertical cross-sectional view of the first exemplary structure according to the first embodiment of the present disclosure after formation of a doped polysilicon emitter.

FIG. 9 is a vertical cross-sectional view of the first exemplary structure according to the first embodiment of the present disclosure after formation of a contact-level dielectric material layer and various contact via structures.

FIG. 10 is a vertical cross-sectional view of a second exemplary structure according to a second embodiment of the present disclosure.

FIG. 11 is a vertical cross-sectional view of a third exemplary structure according to a third embodiment of the present disclosure.

DETAILED DESCRIPTION

As discussed above, the present disclosure relates to vertical polysilicon-germanium heterojunction bipolar transistors and methods of manufacturing the same, which are now described in more detail by referring to the accompanying drawings. Like and corresponding elements are referred to by like reference numerals in the drawings. The drawings are not necessarily drawn to scale.

Referring to FIG. 1, a first exemplary structure according to a first embodiment of the present disclosure includes a substrate 8, which includes a vertical stack, from bottom to top, of a substrate layer 10, a subcollector layer 32L, a collector layer 30L, and a base layer 60L. The entirety of the substrate 8 can be a single crystalline substrate with epitaxial alignment throughout the entirety thereof. The substrate 8 of FIG. 1 can be provided, for example, by providing a single crystalline germanium substrate and implanting dopants of various conductivity types into an upper portion of the single crystalline germanium substrate. For example, dopants of a first conductivity type, which can be p-type or n-type, can be implanted into the single crystalline germanium substrate to form the subcollector layer 32L and the collector layer 30L. Typically, the single crystalline germanium substrate is intrinsic or has a doping of the opposite conductivity type to the first conductivity type, which is herein referred to as a second conductivity type. If the first conductivity type is p-type, the second conductivity type is n-type, and vice versa.

Thus, the substrate layer 10 is either intrinsic or has a doping of the second conductivity type at a dopant concentration, which can be from 1.0×10¹⁴/cm³ to 1.0×10¹⁹/cm³, and typically from 1.0×10¹⁵/cm³ to 1.0×10¹⁶/cm³, although lesser and greater dopant concentrations can also be employed. The subcollector layer 32L has a doping of the first conductivity type at a dopant concentration from 1.0×10¹⁸/cm³ to 1.0×10²¹/cm³, and typically from 1.0×10¹⁹/cm³ to 5.0×10²⁰/cm³, although lesser and greater dopant concentrations can also be employed. The subcollector layer 32L can have a uniform thickness throughout, which can be typically from 100 nm to 1,000 nm, although lesser and greater thicknesses can also be employed. The subcollector layer 32L provides a lateral conductive path. The collector layer 30L has a doping of the first conductivity type at a dopant concentration from 1.0×10¹⁵/cm³ to 5.0×10¹⁹/cm³, and typically from 1.0×10¹⁶/cm³ to 1.0×10¹⁹/cm³, although lesser and greater dopant concentrations can also be employed. The collector layer 30L can have a uniform thickness throughout, which can be typically from 100 nm to 500 nm, although lesser and greater thicknesses can also be employed.

The base layer 60L has a doping of the second conductivity type at a dopant concentration from 1.0×10¹⁶/cm³ to 2.0×10²⁰/cm³, and typically from 1.0×10¹⁷/cm³ to 1.0×10²⁰/cm³, although lesser and greater dopant concentrations can also be employed. The base layer 60L can have a uniform thickness throughout, which can be typically from 30 nm to 300 nm, although lesser and greater thicknesses can also be employed.

While FIG. 1 illustrates an embodiment in which the subcollector layer 32L, the collector layer 30L, and the base layer 60L laterally extend throughout the entirety of the substrate 8, embodiments in which an implantation mask layer (not shown) is employed to form the subcollector layer 32L, the collector layer 30L, and the base layer 60L only within an area of the substrate 8 that is less than the entire area of the substrate 8 can also be employed.

Alternately, one or more of the subcollector layer 32L, the collector layer 30L, and the base layer 60L can be formed by epitaxial deposition of germanium, for example, by chemical vapor deposition employing a precursor such as GeH₄, Ge₂H₆, GeHCl₃, GeH₂Cl₂, and/or any other precursor for depositing germanium as known in the art.

The substrate 8 thus includes a vertical stack, from top to bottom, of a base layer 60L, the collector layer 30L, and the subcollector layer 32L. The collector layer 30L is a first single crystalline germanium layer having a doping of the first conductivity type. The base layer 60L is a second single crystalline germanium layer epitaxially aligned to the collector layer 30L and has a doping of the second conductivity type. The subcollector layer 32L is a third single crystalline germanium layer, has a doping of the first conductivity type, underlies the collector layer 30L, and is epitaxially aligned to the collector layer 30L. Typically, the dopant concentration of the subcollector layer 32L is greater than the dopant concentration of the collector layer 30L.

Each of the subcollector layer 32L and the collector layer 30L consists essentially of germanium and dopants of the first conductivity type. The base layer 60L consists essentially of germanium and dopants of the second conductivity type. P-type dopants include B, Ga, and In. N-type dopants include P, As, and Sb.

Referring to FIG. 2, deep trench isolation structures 20 are formed in the substrate 8. The deep trench isolation structures 20 have dielectric outer surfaces. The deep trench isolation structures 20 can be formed by forming deep trenches extending from the top surface of the substrate 8 to a depth greater than the depth of the bottom surface of the subcollector layer 32L, and subsequently filling the deep trenches with a dielectric material or a combination of a dielectric liner and another fill material. For example, the dielectric material or the dielectric liner can be silicon oxide, silicon oxynitride, and/or silicon nitride. If another fill material is employed in combination with a dielectric liner, the fill material is deposited after the dielectric liner is deposited. Exemplary fill materials include, but are not limited to, polysilicon, polycrystalline silicon-germanium alloys, and any dielectric material.

The deep trench isolation structures 20 laterally enclose portions of the base layer 60L, the collector layer 30L, the subcollector layer 32L, and an uppermost portions of the substrate layer 10 within an area, which is herein referred to as a heterojunction bipolar transistor region HBTA. The portion of the subcollector layer 32L laterally surrounded by a deep trench isolation structure 20 and located within the heterojunction bipolar transistor region HBTA is a subcollector 32A. The portion of the collector layer 30L laterally surrounded by a deep trench isolation structure 20 and located within the heterojunction bipolar transistor region HBTA is a collector 30A. The portion of the base layer 60L laterally surrounded by a deep trench isolation structure 20 and located within the heterojunction bipolar transistor region HBTA is a base structure region 60′.

The portion of the subcollector layer 32L located outside the heterojunction bipolar transistor area HBTA is herein referred to as a miscellaneous subcollector-level structure 32B, which may, or may not, be a part of another semiconductor structure (such as another heterojunction bipolar transistor (HBT) to be formed). The portion of the collector layer 30L located outside the heterojunction bipolar transistor area HBTA is herein referred to as a miscellaneous collector-level structure 30B, which may, or may not, be a part of another semiconductor structure. The portion of the base layer 60L located outside the heterojunction bipolar transistor area HBTA is herein referred to as a miscellaneous base-level structure 60″, which may, or may not, be a part of another semiconductor structure.

The collector 30A is a first single crystalline germanium portion having a doping of the first conductivity type. The base structure region 60′ includes a second single crystalline germanium portion epitaxially aligned to the collector 30A, and has a doping of the second conductivity type. The subcollector 32A includes a third single crystalline germanium portion epitaxially aligned to the collector 30A, and has a doping of the first conductivity type. Typically, the dopant concentration of the subcollector 32A is greater than the dopant concentration of the collector 30A.

Referring to FIG. 3, shallow trench isolation structures 22 are formed in an upper portion of the substrate. The shallow trench isolation structures 22 are formed in areas that include areas within and around the stack of the collector 30A and the base structure region 60′. Specifically, the shallow trench isolation structures 22 can be formed, for example, by etching shallow trenches in the base layer 60L and in the upper portion of the collector layer 30L in areas that overlie the deep trench isolation structures 20 and across a portion of the heterojunction bipolar transistor area HBTA. The shallow trenches are filled with a dielectric material such as silicon oxide, silicon nitride, and/or silicon nitride, and the excess dielectric material above the top surface of the substrate 8 is removed, for example, by planarization such as chemical mechanical planarization (CMP).

The base structure region 60′ is divided into a base 60A and a base-level feedthrough region 60Z by a portion of the shallow trench isolation structures 22. The base 60A is formed within an emitter area EA, and the base-level feedthrough region 60Z is formed within a feedthrough area FA. The emitter area EA and the feedthrough area FA do not overlap, and are located within the heterojunction bipolar transistor area HBTA. Each of the base 60A and the base-level feedthrough region 60Z is laterally surrounded by the shallow trench isolation structures 22. The subcollector 32A, the collector 30A, and the base 60A include single crystalline germanium portions that are epitaxially aligned among one another, and are formed within the substrate 8. The base 60A has the same thickness throughout.

Referring to FIG. 4, a first block level photoresist 57 is applied to the top surface of the substrate 8 and is lithographically patterned to form an opening in the feedthrough area FA. Dopants of the first conductivity type are implanted into the base-level feedthrough region 60Z and a portion of the collector 30A to form a subcollector feedthrough region 31. The subcollector feedthrough region 31 has a doping of the first conductivity type, and extends from the top surface of the substrate 8 to the top surface of the subcollector 32A to provide an electrically conductive path. Thus, the top surface of the subcollector feedthrough region 31 is coplanar with the top surface of the base 60A. The concentration of dopants of the first conductivity type within the subcollector feedthrough region 31 can be from 1.0×10¹⁸/cm³ to 1.0×10²¹/cm³, and typically from 1.0×10¹⁹/cm³ to 5.0×10²⁰/cm³, although lesser and greater dopant concentrations can also be employed. The only requirement is that the concentration of dopants of the first conductivity type within the subcollector feedthrough 31 is greater than the concentration of dopants of the second type in the base-level feedthrough region 60Z, resulting in the region 60Z being converted from second conductivity type to first conductivity type. The first block level photoresist 57 is subsequently removed, for example, by ashing.

Referring to FIG. 5, an extrinsic base layer 61L and an extrinsic base cap layer 62L are deposited over the substrate 8. The extrinsic base layer 61L is deposited as a blanket layer having a uniform thickness directly on the top surface of the substrate 8, for example, by chemical vapor deposition. The extrinsic base layer 61L includes a polycrystalline semiconductor material having a doping of the second conductivity type. For example, the extrinsic base layer 61 can include polysilicon, germanium, or a silicon-germanium alloy and dopants of the second conductivity type. The thickness of the extrinsic base layer 61L can be from 20 nm to 300 nm, and typically from 50 nm to 150 nm, although lesser and greater thicknesses can also be employed.

The extrinsic base cap layer 62L is deposited as a blanket layer having a uniform thickness directly on the top surface of the extrinsic base layer 61L, for example, by chemical vapor deposition. The extrinsic base cap layer 61L includes a dielectric material such as silicon oxide, silicon nitride, and/or silicon oxynitride. The thickness of the extrinsic base cap layer 62L can be from 15 nm to 150 nm, and typically from 30 nm to 100 nm, although lesser and greater thicknesses can also be employed.

Referring to FIG. 6, a second block level photoresist 67 is applied over the stack of the extrinsic base layer 61L and the extrinsic base cap layer 62L. The second block level photoresist 67 is lithographically patterned to form an opening in a center portion of the emitter area EA. The second block level photoresist 67 covers a peripheral portion of the emitter area EA, which is laterally surrounded by the shallow trench isolation structure 22.

An anisotropic etch can be employed to remove the portions of the extrinsic base cap layer 62L and the portions of the extrinsic base layer 61L that are not covered by the patterned second block level photoresist 67. A remaining portion of the extrinsic base cap layer 62L at a periphery of the emitter area EA forms an extrinsic base cap 62, and a remaining portion of the extrinsic base layer 61L around the periphery of the emitter area EA forms an extrinsic base 60B. The sidewalls of the extrinsic base 60B and the sidewalls of the extrinsic base cap 62 are vertically coincident, i.e., coincident when viewed in a vertical direction. The second block level photoresist 67 is subsequently removed, for example, by ashing.

The extrinsic base 60B includes a polycrystalline semiconductor material having a doping of the second conductivity type. The extrinsic base 60B contacts a peripheral portion of the top surface of the base 60A to provide an electrical contact to the base 60A.

Referring to FIG. 7, dielectric spacers 76 are formed on the inner sidewall and the outer sidewall of the stack of the extrinsic base 60B and the extrinsic base cap 62. The dielectric spacers 76 can be formed by deposition of a conformal dielectric material layer and an anisotropic etch that removes horizontal portions of the conformal dielectric material layer. The remaining vertical portions of the conformal dielectric material layer after the anisotropic etch form the dielectric spacers 76. The dielectric spacers 76 include a dielectric material such as silicon oxide, silicon nitride, silicon oxynitride, or a combination thereof. The thickness of the dielectric spacers 76, as measured laterally at a bottom portion that contacts the base 60A or the shallow trench isolation structure 22, can be from 10 nm to 100 nm, although lesser and greater thicknesses can also be employed.

Referring to FIG. 8, a doped polysilicon emitter 70 is formed directly on the exposed top surface of the base 60A and a dielectric spacer 76 located on the inner sidewalls of the stack of the extrinsic base 60B and the extrinsic base cap 62. The doped polysilicon emitter 70 has a doping of the first conductivity type, which is the opposite type of the doping of the base 60A and the extrinsic base 60B. The doped polysilicon emitter 70 can consist essentially of polycrystalline silicon and dopants of the first conductivity type.

To form the doped polysilicon emitter 70, a doped polysilicon layer can be deposited as a blanket layer, for example, by chemical vapor deposition. A photoresist (not shown) is applied and lithographically patterned to cover an area of the doped polysilicon layer that includes the area of the interface between the base 60A and the doped polysilicon layer. An etch is employed to remove the portions of the doped polysilicon layer that is not covered by the photoresist. The photoresist is subsequently removed.

The concentration of dopants of the first conductivity type in the emitter can be from 1.0×10¹⁹/cm³ to 2.0×10²¹/cm³, and typically from 1.0×10²⁰/cm³ to 1.0×10²¹/cm³, although lesser and greater dopant concentrations can also be employed. The thickness of the emitter, as measured from the interface between the base 60A and the emitter 70 to a planar top surface of the emitter that is most proximal to the interface, can be from 50 nm to 500 nm, and typically from 100 nm to 300 nm, although lesser and greater thicknesses can also be employed.

Referring to FIG. 9, a contact-level dielectric material layer 90 is deposited over the substrate 8, the stack of the extrinsic base 60B and the extrinsic base cap 62, and the emitter 70. The contact-level dielectric material layer 90 includes a dielectric material such as silicon oxide, silicon nitride, organosilicate glass, or any other dielectric material that can be employed for forming metal interconnect structures as known in the art. For example, the contact-level dielectric material layer 90 can be deposited by chemical vapor deposition or spin coating.

Various contact via structures are formed in the contact-level dielectric material layer 90L to provide electrical contacts to the collector 30A, the base 60A, and the emitter 70. Optionally, metal-semiconductor alloy portions such as a collector-side metal-semiconductor alloy portion 84, a base-side metal-semiconductor alloy portion 86, and an emitter-side metal-semiconductor alloy portion 87 can be formed after formation of via holes in the contact-level dielectric material layer 90 and through the extrinsic base cap 62. The metal-semiconductor alloy portions (84, 86, 87) can be formed, for example, by deposition of a metal layer, formation of metal silicides and metal germanides during an anneal at an elevated temperature, and removal of unreacted portions of the metal layer. The collector-side metal-semiconductor alloy portion 84 is a metal germanide, the emitter-side metal-semiconductor alloy portion 87 is a metal silicide, and the base-side metal-semiconductor alloy portion 86 can be a metal silicide, a metal germanide, or a metal germanosilicide depending on the composition of the doped semiconductor material in the extrinsic base 60B.

The various contact via structures can be formed by filling the via holes in the contact-level dielectric material layer 90 with a conductive material followed by removal of excess conductive material above the contact-level dielectric material layer 90. The various contact via structures include an emitter contact via structure 97 conductively connected to the emitter 70 and embedded in the contact-level dielectric material layer 90, a base contact via structure 96 conductively connected to the base 60A and embedded in the contact-level dielectric material layer 90; and a collector contact via structure 94 conductively connected to the collector 30A and embedded in the contact-level dielectric material layer 90. As used herein, a first element is conductively connected to a second element if there exists a conductive current path between the first element and the second element. Specifically, the emitter contact via structure 97 directly contacts the emitter 70 or is conductively connected to the emitted 70 via the emitter-side metal-semiconductor alloy portion 87. The base contact via structure 96 is conductively connected to the base 60A through the extrinsic base 60B and optionally through the base-side metal-semiconductor alloy portion 86. The collector contact via structure 94 is conductively connected to the collector 30A via the subcollector 32A and the subcollector feedthrough region 31 and optionally through the collector-side metal-semiconductor alloy portion 84.

The first exemplary structure includes a heterojunction bipolar transistor (HBT) structure in which the emitter 70 and the base 60A have different semiconductor materials. In the operation of bipolar transistor, for a given emitter-base forward bias, the collector current is proportional to exp(−Eg_(base)/kT), where Eg_(base) is the energy bandgap of the base region. Thus, a small base bandgap leads to large collector current. The base current is proportional to exp(−Eg_(emitter)/kT), where Eg_(emitter) is the energy bandgap of the emitter region. Thus a large emitter energy bandgap leads to small base current. Germanium, with a bandgap of 0.66 eV, is excellent as the narrow-gap base material. Polysilicon, with a bandgap of about 1.12 eV and compatible with modern very large scale integration (VLSI) technology, is an excellent wide-gap emitter material.

The heterojunction bipolar transistor (HBT) of the present disclosure employs a wide-gap emitter and a narrow-gap base. Specifically, the HBT of the present disclosure employs a single crystalline germanium base to provide a narrow-gap base and a polysilicon emitter to provide a wide-gap emitter. The HBT of the present disclosure is superior for low-voltage operation in comparison with prior art bipolar transistors because of the differences in the bandgap between the material of the base and the material of the emitter.

As an example, consider trying to obtain a collector current density of 10 mA/μm². A usual bipolar transistor having a single crystalline silicon base and a polysilicon emitter needs a emitter-base forward bias of 0.92 V. If the silicon base is replaced with a single crystalline SiGe alloy with 20% of Ge in atomic concentration to provide a usual SiGe-base transistor having an energy bandgap of 200 meV smaller than the energy bandgap of silicon, the required emitter-base bias is about 0.72 V. For the HBT of the present disclosure having a single crystalline germanium base and a polysilicon emitter, the corresponding voltage required for providing collector current density of 10 mA/μm² is only 0.46 V.

Referring to FIG. 10, a second exemplary structure according to a second embodiment of the present disclosure can be formed by employing a germanium-on-insulator (GOI) substrate 8′ instead of the substrate 8 of the first exemplary semiconductor structure. The GOI substrate 8′ includes a stack of a substrate layer 10, a buried insulator layer 12 that includes a dielectric material such as silicon oxide, silicon nitride, and/or silicon oxynitride, and a singe crystalline germanium layer. A subcollector layer 32L, a collector layer 30L, and a base layer 60L are formed employing the same methods as the methods employed at the step corresponding to FIG. 1 of the first embodiment. Subsequently, processing steps of FIGS. 2-9 can be employed to provide the second exemplary structure. In the second exemplary structure, a bottom surface of the subcollector 32A contacts the top surface of a buried insulator layer 12 within the GOI substrate 8′. The HBT structure in the second exemplary structure can provide the same functional advantage as the HBT structure in the first exemplary structure in addition to providing enhanced electrical isolation from adjacent devices and the substrate layer 10 due to the presence of the buried insulator layer 12.

Referring to FIG. 11, a third exemplary structure according to a third embodiment of the present disclosure is derived from the second exemplary structure by omitting the formation of a subcollector layer 32L at the processing step corresponding to the processing step of FIG. 1. Thus, the bottom surface of the collector 30A contacts the top surface of a buried insulator layer 12 within the GOI substrate 8′. The HBT structure in the third exemplary structure can provide the same functional advantage as the HBT structure in the first exemplary structure in addition to providing enhanced electrical isolation from adjacent devices and the substrate layer 10 due to the presence of the buried insulator layer 12. The HBT structure in the third exemplary structure simplifies the manufacturing process by eliminating the subcollector 32A, but the resistance between the collector contact via structure 94 and the collector 30A can be greater in the third exemplary structure than in the second exemplary structure.

While this disclosure has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the present disclosure. It is therefore intended that the present disclosure not be limited to the exact forms and details described and illustrated, but fall within the scope of the appended claims. 

1. A heterojunction bipolar transistor (HBT) structure comprising: a collector comprising a first single crystalline germanium portion having a doping of a first conductivity type; a base comprising a second single crystalline germanium portion epitaxially aligned to said collector and having a doping of a second conductivity type that is the opposite of said first conductivity type; and an emitter comprising a polycrystalline silicon portion having a doping of said first conductivity type.
 2. The HBT structure of claim 1, wherein said second single crystalline germanium portion consists essentially of germanium and dopants of said second conductivity type.
 3. The HBT structure of claim 1, wherein said polycrystalline silicon portion consists essentially of polycrystalline silicon and dopants of said first conductivity type.
 4. The HBT structure of claim 1, wherein said base has a same thickness throughout said second single crystalline germanium portion.
 5. The HBT structure of claim 1, wherein said base and said collector are located in a substrate, and said HBT structure further comprises a shallow trench isolation structure laterally surrounding said base.
 6. The HBT structure of claim 5, further comprising a deep trench isolation structure extending from a bottom surface of said shallow trench isolation structure to a depth below a bottom surface of said collector, wherein said deep trench isolation structure laterally surrounds said collector.
 7. The HBT structure of claim 1, further comprising an extrinsic base comprising a polycrystalline semiconductor material having a doping of said second conductivity type and contacting a peripheral portion of a top surface of said base.
 8. The HBT structure of claim 7, further comprising a dielectric spacer contacting a sidewall of said extrinsic base and a sidewall of said emitter.
 9. The HBT structure of claim 1, further comprising: a contact-level dielectric material layer overlying said emitter, said base, and said collector; an emitter contact via structure conductively connected to said emitter and embedded in said contact-level dielectric material layer; a base contact via structure conductively connected to said base and embedded in said contact-level dielectric material layer; and a collector contact via structure conductively connected to said collector and embedded in said contact-level dielectric material layer.
 10. The HBT structure of claim 1, further comprising a subcollector comprising a third single crystalline germanium portion and having a doping of said first conductivity type and underlying said collector, wherein a dopant concentration of said subcollector is greater than a dopant concentration of said collector.
 11. The HBT structure of claim 10, further comprising a subcollector feedthrough region extending from said subcollector to a surface that is coplanar with a top surface of said base.
 12. The HBT structure of claim 1, wherein said base and said collector are located in a single crystalline germanium substrate, and a horizontal top surface of said base is a top surface of said single crystalline germanium substrate.
 13. The HBT structure of claim 1, wherein said base and said collector are located in a germanium-on-insulator (GOI) substrate.
 14. The HBT structure of claim 13, further comprising a subcollector comprising a third single crystalline germanium portion and having a doping of said first conductivity type and underlying said collector, wherein a bottom surface of said subcollector contacts a top surface of a buried insulator layer within said GOI substrate.
 15. The HBT structure of claim 13, wherein a bottom surface of said collector contacts a top surface of a buried insulator layer within said GOI substrate.
 16. A method of fabricating a heterojunction bipolar transistor (HBT) structure, comprising: forming a collector and a base in a substrate, wherein said collector comprises a first single crystalline germanium portion having a doping of a first conductivity type and said base comprises a second single crystalline germanium portion epitaxially aligned to said collector and has a doping of a second conductivity type that is the opposite of said first conductivity type; and forming an emitter comprising a polycrystalline silicon portion having a doping of said first conductivity type directly on said base.
 17. The method of claim 16, wherein said second single crystalline germanium portion consists essentially of germanium and dopants of said second conductivity type.
 18. The method of claim 16, wherein said polycrystalline silicon portion consists essentially of polycrystalline silicon and dopants of said first conductivity type.
 19. The method of claim 16, further comprising: forming a stack of a collector layer and a base layer, wherein said collector layer is a first single crystalline germanium layer having a doping of said first conductivity type and said base layer is a second single crystalline germanium layer epitaxially aligned to said collector layer and has a doping of said second conductivity type; and forming a deep trench isolation structure laterally surrounding a stack of a portion of said collector layer and a portion of said base layer.
 20. The method of claim 19, further comprising forming a shallow trench isolation structure within and around said stack of said portion of said collector layer and said portion of said base layer, wherein a remaining portion of said base layer includes said base and a remaining portion of said collector layer includes said collector.
 21. The method of claim 16, further comprising forming an extrinsic base comprising a polycrystalline semiconductor material having a doping of said second conductivity type directly on a peripheral portion of a top surface of said base.
 22. The method of claim 21, further comprising forming a dielectric spacer directly on a sidewall of said extrinsic base, wherein said emitter is formed on another sidewall of said dielectric spacer.
 23. The method of claim 16, further comprising forming a subcollector comprising a third single crystalline germanium portion and having a doping of said first conductivity type in a substrate, wherein said collector contacts said subcollector, and a dopant concentration of said subcollector is greater than a dopant concentration of said collector.
 24. The method of claim 23, further comprising forming a subcollector feedthrough region by implanting dopants of said first conductivity type into a germanium-containing portion, wherein said collector feedthrough contacts said subcollector.
 25. The method of claim 16, wherein said base and said collector are formed in a germanium-on-insulator (GOI) substrate. 